(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming tungsten layers for conductive interconnects.
(2) Description of Prior Art
The use of tungsten in the fabricating of very-large-scale-integrated(VLSI) circuits has been practiced since the mid 1980s. As a conductive material tungsten does not rank as high as aluminum, which has been the primary conductor used in micro-circuit chip technology for nearly forty years. On the other hand, tungsten provides many features which make it an important material for fabricating metal-to-silicon contacts, via plugs, and for intra level interconnects. In this capacity tungsten is deposited into contact or via openings, and anisotropically etched to the insulating layer, leaving tungsten in the openings. Where local interconnects are desired the deposited tungsten layer is patterned with photoresist and anisotropically etched, leaving interconnecting stripes over the insulator.
Chemical-vapor-deposited(CVD) tungsten has proven to be an excellent material for such interconnect applications because of its low resistance, low stress(less than 5.times.10.sup.9 dynes/cm.sup.2), and a coefficient of thermal expansion which closely matches that of silicon. In addition tungsten has a high resistance to electromigration which is a common problem with aluminum its alloys. CVD tungsten can be deposited at temperatures around 400.degree. C. with good conformity and step coverage.
Although tungsten does not bond well to either silicon or to the adjacent silica based insulating layer, a thin layer(less than 1,000 Angstroms) of titanium(Ti) is often used as a bonding agent to the silicon. This is covered by a layer of titanium nitride(TiN) which acts as a diffusion barrier as well as a bonding layer to the tungsten. The Ti layer, when thermally annealed, fuses with silicon to form titanium silicide(TiSi.sub.2) and with the silica based insulating layer to form a titanium silicate(Ti.sub.x SiO.sub.y). Adhesion of the TiN to the Ti and subsequently to the tungsten is considered excellent.
The good conformity and step coverage afforded by tungsten is due in large part to the nature of the deposition process itself. In the CVD process, particularly with low-pressure-chemical-vapor-deposition(LPCVD), the chemical reaction which forms the product occurs at the heated surface of the material receiving the deposition. At higher pressures, reaction tends to occur in the gas phase with an accompanying loss of step coverage.
Step coverage is a measure of how well a deposited layer maintains its nominal thickness as it crosses a step. FIG. 1 (adapted from Wolf, S., "Silicon Processing for the VLSI Era", Vol.2, Lattice Press, Sunset Beach, Calif., (1990), p.202, illustrates this measure. A layer of material 14 of nominal thickness t.sub.n is deposited onto a substrate material 10 having a step 16 typically formed in a deposited layer 12. The step coverage is defined by the minimum thickness of the film t.sub.s divided by its nominal thickness t.sub.n expressed in percent.
Step coverage becomes an important consideration in sub-micron integrated circuit technology where openings having high aspect ratios are frequently encountered, particularly in the formation of device level contacts. The aspect ratio is defined as the ratio of the depth of the opening to its width. Contact openings having aspect ratios of 3 or greater are becoming commonplace as device dimensions continue to shrink.
The early CVD deposition processes for tungsten required temperatures above 500.degree. C. in order to achieve acceptable layers with good edge coverage and resistivity. Sub-micron, integrated circuit elements are prone to damaging profile distortions when subjected to such temperatures and therefore have a restricted thermal budget.
One of the problems associated with the CVD deposition of tungsten involves the granular structure of the deposited material. Although low pressure techniques produce films having good edge coverage, the large grain structure results if films with poor specular reflectivity, a higher degree of stress, and a higher resistivity. The specular reflectivity is an indication of the granular structure of the film.
Chang, et. al., U.S. Pat. No. 5,028,565 cites that tungsten layers having good adhesion, a high specular reflectivity as well as a high deposition rate can be deposited at a temperatures between 350.degree. C. and 525.degree. C. using high pressures and a two step procedure. A nucleation layer of about 500 Angstroms is first deposited using the SiH.sub.4, reduction of WF.sub.6 at 450.degree. C. and a pressure of 80 Torr. The main tungsten layer is then formed using the hydrogen reduction of WF.sub.6. Raising the deposition temperature to 475.degree. C. resulted in a lowering of stress. The presence of nitrogen in the carrier gas had a beneficial effect on the reflectivity.
Emesh U.S. Pat. No. 5,407,698 confirms the observations of Chang but states the method is not practical because many known CVD reactors are incapable of operating at such high pressures. Emesh, operating at lower pressures, introduces a third step in the formation of a tungsten layer. Nucleation is performed by the hydrogen reduction of WF.sub.6 at 100 mTorr and 480.degree. C. where good adhesion is obtained. The carrier gas is entirely of nitrogen. An intermediate step, using a low H.sub.2 /WF.sub.6 ratio, is introduced to fill contact or via openings. Here, at a pressure of 9 Torr, good step coverage is obtained at the expense of increased grain size and high stress. Finally, in a third step, using a high H.sub.2 /WF.sub.6 ratio, good step coverage is sacrificed in order to obtain small grain size, high specular reflectivity, low resistivity, and low stress.
Huang, et.al., U.S. Pat. No. 5,462,890 forms a tungsten plug in a contact or via opening by a two-step deposition process without the use of an adhesion layer such as TiN. The initial tungsten deposition takes place by introducing a reactive gas containing tungsten into a hydrogen radical plasma wherein good adhesion is obtained. The thicker tungsten blanket is then deposited in-situ by means of CVD.